Optical filters are already known that comprise a support layer having metal reflective elements formed thereon to define a grating of slits smaller than the wavelength that is to be filtered. The amplitude of transmission is adjusted both by the width and by the thickness of the slits, with the thickness being selected to be of the same order as but less than half the wavelength under consideration.
That type of filter is described in particular in the articles by J. A. Porto et al. “Transmission resonances on metallic gratings with very narrow slits”, Physical Review Letters, 83, p. 2845, 1999, and by G. Vincent et al. “Large-area dielectric and metallic freestanding gratings for mid-infrared optical filtering application”, Journal of Vacuum Science Technology, B 26, p. 852, 2008.
Thus, those types of filter require thick reflective elements that are sometimes suspended directly above air, so that the refractive index of the support is as close as possible to that of the incident medium (air/vacuum).
Under such conditions, such an optical filter is technologically difficult to make.
Furthermore, the spectral range over which good rejection is ensured around the main transmission peak is very limited.
Another optical filter is known that comprises reflective elements forming a conventional grating of slits, a halfwave plate supporting the grating and a medium in contact with the halfwave plate and presenting relative thereto a refractive index contrast that is as great as possible.
Such an optical filter is described in particular in Document FR 2 959 021.
With such a filter, the halfwave plate forms a waveguide under the grating of slits, the grating also being capable of exciting plasmon surface modes. The combination of the grating, of the plate, and of the large refractive index contrast makes it possible to form an electromagnetic resonator that effectively traps light in the plate before it escapes via the medium in contact with the plate.
Furthermore, that optical filter is simpler to fabricate than the suspended optical filter.
Nevertheless that filter presents certain drawbacks.
In particular, although its dimensions can be selected so as to optimize transmission at a given wavelength, the transmission spectrum of the filter presents significant peaks away from that wavelength.
This leads to poor rejection outside a rather narrow wavelength window.
That puts limitations on the use of the optical filter.
Thus, when the filter is used for hyperspectral imaging, the spectral range of interest is limited. Interference can occur between pixels of different colors, thereby complicating the color reconstruction of the image.
Also, when used in a gas sensor, it is important to distinguish spectrally between weak signals that are specific to different species. Unfortunately, the limitations of the optical filter lead to a degraded signal-to-noise ratio for detection.